The present invention relates to a self-propelled, unmanned, automatically guided vehicle and, more particularly, to a vehicle which enables all-directional travel and which can be favorably used in a narrow place or a place having an uneven or undulating floor surface as is often present in a semiconductor manufacturing line and which enables carrying of a substance such as a semiconductor wafer or the like while substantially preventing vibration of the carried substance.
As a prior art traveling mechanism, there is disclosed a vehicle capable of all-directional travel as described in Japanese Patent Application Laid-Open No. 149270/1988. In this vehicle, since four wheels are arranged orthogonally and are provided with rollers having a rotational axis in the circumferential direction of the outer circumferential surface of the wheel and a reduction gear and a motor fixed to the body are utilized, movement in the traveling direction of the wheel and in the rotational direction of the roller perpendicular to the traveling direction of the wheels is possible.
In this system, however, a problem exists in that when the ground or surface contact of the four wheels with the travel surface is insecure, the straight running of the vehicle becomes difficult. That is, even if slight unevenness exists on the road surface, the state may be easily produced in that three wheels among the four wheels in the longitudinal and lateral directions are grounded stably and one wheel is not grounded, i.e., does not make proper ground contact. In this case, in order that the wheels arranged in the lateral direction are driven and the wheels in the longitudinal direction are in a stationary state and the body travels forward, if one of the lateral wheels during the driving is not grounded, the driving force of the wheels is generated in dissymetry with respect to the body. The elimination of the partial distribution of the driving force in this case is dependent upon the anti-slip force in a direction other than the rotational direction of the rollers in the longitudinal wheels. However, since the rollers in the longitudinal wheels are rotated for the forward advance, generation of the slip in the lateral direction is inevitable due to slight deformation of the roller surface and therefore the traveling direction of the body is not stabilized. Additionally, when four or more wheels are used, the wheels must follow the unevenness of the road surface.
Further, since motion of the body on a plane has only three degrees of freedom, x, y, .theta. essentially, when utilizing four or more driving wheels, measures must be taken in that a driving mechanism using a differential gear mechanism is constituted, thereby a redundant degree of freedom is eliminated, or in that the circumferential speed of each wheel is strictly coincident. Otherwise, slip is inevitable. In the former measure, the mechanism is complicated and the mass of the body is increased. Consequently, this is generally unsuitable for a self-propelled vehicle powered by a battery. In the latter measure, the rotational speed of the motor is strictly controlled or the reduction ratio of the motor must be decreased so that the circumferential speed of the wheel can be easily varied by an external force. In the strict control of the circumferential speed of the wheel, the driving system of each wheel must be adjusted over the full rotational speed range and this control is quite difficult. On the other hand, when the reduction ratio of the motor is decreased, a problem exists in that the dynamic characteristics of the self-propelled vehicle as a whole are deteriorated.
The aforementioned prior art does not take into consideration the proper carrying of a substance. However, Japanese Patent Application Laid-Open No. 135485/1989, for example, discloses a device wherein a substance to be carried is set on a substance mounting bed and four corners are guided so that the substance is always held to a definite attitude, but such does not take into consideration actively preventing vibration of the substrate.
In the prior art as above described, generation of large amount of vibration cannot be avoided depending on the traveling speed of the self-propelled vehicle and the state of the traveling surface. Particularly, when a substrate carried by the self-propelled vehicle is wafer in a semiconductor manufacturing process, an outer edge portion of the wafer may be chipped by vibration during carrying or the wafer may be rubbed with a periphery to cause generation of dust and contamination of the wafer. In a semiconductor manufacturing process, the wafer, during manufacturing, is usually carried in an enclosed state in a cassette made of tetrafluoroethylene or polypropylene. Although such material is soft, and therefore is not liable to damage, the wafer and also is not liable to be abraded as an organic material, abrasion may be produced. In the semiconductor manufacturing process, since even a small amount of abrasion powder may be cause a defective state, sliding must be avoided as far as possible. However, such material has a low coefficient of friction, and the wafer is moved on the mounting base even with slight vibration acceleration. For example, in the case of tetrafluoroethylene, the static friction coefficient with metal at a low load in the atmosphere is about 0.1. Consequently, when tetrafluoroethylene is used in the mounting base of the wafer, the vibration acceleration must be suppressed to 0.1 G (about 1 m/s.sup.2) or less as follows: EQU F.sub.1 =.alpha.M EQU F.sub.2 =.mu.Mg
Now, from F.sub.1 .ltoreq.=F.sub.2 EQU .alpha.M=.mu.Mg EQU .thrfore..alpha..ltoreq..mu.g
where
F.sub.1 : force to cause sliding PA1 F.sub.2 : force to prevent sliding PA1 .alpha.:vibration acceleration PA1 M: mass of wafer PA1 .mu.: static friction coefficient PA1 g: gravitational acceleration
In the case of a self-propelled vehicle of the prior art, even if the road surface state of the traveling load is good, the vibration acceleration of about 0.3 G (about 3 m/s.sup.2) is applied to the substrate to be carried. Further, if the road surface state is bad, the vibration acceleration from 0.7 G (amount 7 m/s.sup.2) to 1 G (about 10 m/s.sup.2) or more may be applied to the substance to be carried. Thus, in the prior art, the carrying, particularly requiring avoidance of vibration, such as wafer carrying in a semiconductor manufacturing process is not sufficiently considered, and a problem exists in that impact is applied to the substance to be carried and damage is produced. Particularly, when the substance to be carried is a semiconductor wafer or the like, a problem exists in that generation of dust due to the abrasion of the mounting base or the like causes a defective state of the wafer.